1. The present invention relates to an abrasive composition. More particularly, the present invention relates to an abrasive composition for rapidly polishing or providing a mirror finish to an aluminum-based substrate for a magnetic recording disc, and a process therefor.
2. Description of the Related Art
A magnetic recording disc comprises an aluminum-based substrate including an aluminum or aluminum alloy (referred to only as aluminum hereafter) disc, a nickel substrate formed by plating nickel-phosphine onto an aluminum disc, and an anodized aluminum substrate formed by anodizing an aluminum disc. A magnetic film is applied to the aluminum-based substrate by coating, plating or sputtering, and the surface of the aluminum-based substrate is provided with a mirror-finish by polishing or lapping.
A known abrasive composition used for polishing an aluminum substrate for a magnetic recording disc is a slurry which is a mixture of an aluminum abrasive and an aqueous solution of a basic or alukaline etchant. For example, H. Kitano et al., in Japanese Unexamined Patent Publication (Kokai) No. 61-278587, published on Dec. 9, 1986, disclose an abrasive composition comprising an aluminous abrasive, nickel sulfate, and water and T. Senda et al., in Japanese Unexamined Patent Publication (Kokai) No. 62-25187, published on Feb. 3, 1987 disclose an abrasive composition comprising an alumina substrate, water, and nickel or aluminum nitride, or a combination of nickel nitride or sulfate with aluminum nitride.
Due to recent demands for a denser recording on a magnetic recording disc, the aluminum-based substrate must have a finer polished surface with fewer pits, protrusions and scratches. Further, due to the recent remarkable fall in the price of a magnetic recording disc, a high polishing efficiency is necessary to reduce the working or processing cost, which is a relatively large factor in the cost of producing a magnetic recording disc. Known weak acid-based abrasive compositions, however, cannot meet these requirements, and strong acid-based abrasive ,compositions have a high polishing efficiency but are limited in use because of the accompanying problem of the disposal treatment of a strong acid radical such as nitric acid radical.
The main object of the present invention is to meet the above requirements and to provide an abrasive composition able to provide an aluminum-based substrate with a very fine finished surface at a high efficiency.